It is previously known to store petroleum products, and also other liquids lighter than water, in a cavity formed in groundwater carrying rock formations, in which the stored liquid lies in direct contact with the water-permeable surface of the cavity walls. The pressure exerted on the cavity walls by the liquid stored in the cavity is lighter than the pressure exerted by surrounding groundwater, thereby counteracting any tendency of the stored liquid to pass through the wall.
When the stored liquid is lighter than water and insoluble therein, it is a normal practice to provide a water bed in the lowermost region of the cavity.
SE-A-7802027-8 and 7901278-7 describe and illustrate complexes for storing petroleum products and other fluids in rock formations. These storage complexes, or locations, have a very high storage capacity, despite being of relatively small horizontal extension. The stored product is therewith located within a concentrated area, and the expedient of shielding the storage area with a curtain of densely packed, waterfilled drill holes can therefore be more readily carried out, thereby to off-set lowering of the groundwater level and preventing the stored product from spreading to the complex surroundings.
According to these patent specifications, the cavities are located at substantially mutually equal depths, and when seen in horizontal section each cavity has a substantially circular or oval shape, and when seen in horizontal cross-section throughout the whole of the complex the mean points of the circular or oval horizontal sections of respective cavities lie in the corners of regular polygons, all having the same number of sides.
By regular polygon is meant a polygon in which all sides are of mutually equal length and all corner angles are the same. A regular polygon can thus be inscribed in a circle which passes through all of the corner points and the centre of which thus also forms the centre of the polygon.
In accordance with one embodiment of the invention these polygons have the form of various sized pentagons having a common centre point. The cavities are therefore arranged in concentric circles. A further cavity can be arranged so that its centre axis coincides with the centre point of these circles.
It is also known from SE-A-8300185-9 to construct in rock formations a fluid-storage cavity location in which the actual cavity has the form of a substantially vertical cylinder, around which there is provided a series of vertical holes forming a water-drainage shield; this drainage shield is intended for removal of the water bed upon which the stored fluid has rested.
Present day rock cavities for storing oil or petroleum products have the form of long "loaves", i.e. horizontally extending rock cavities presenting a bottom surface area of 500.times.25 m or thereabove, and a height of 30 m. It has been found that when storing oil products in rock cavities of this kind, in which the oil rests on a bed of water, microorganisms grow in the boundary layer between water and oil, the oil/oil products being destroyed thereby and rendered worthless for future use. When such cavities are used to store refined products, it has been found necessary to re-refine the products in order to guarantee their usefulness.
In order to overcome these problems associated with horizontal cavities, the use of vertical, substantially cylindrical rock cavities has been proposed, as beforementioned. Examples of such vertical cavities are described and illustrated in SE-A-7901278-7 and SE-A-8300185-9, and in subsequent articles by K. I. Sagefors et al, WP-System, Stockholm, Sweden. When excavating the rock in the construction of such vertical cavities, there is first formed a top tunnel from which the conically shaped roof cupola is taken, by first drilling holes oliquely outwards and downwards in the vicinity of the peripheral surface of the ultimate cupola or roof structure, filling these holes with explosives and blasting the rock; forming one or more transport tunnels so that said tunnels open into the cylindrical peripheral surface of the ultimate vertical rock cavity, excavation of rock being effected from the transport tunnels by vertical drilling and bench excavation operations, the shot rock-mass being taken-out at the bottom, which may taper conically downwards to a separate removal tunnel, which can be used for introducing piping and like conduits into the cavity site, and for removing goods stored in the cavity.
As mentioned above, previous methods for constructing substantially cylindrical, vertical cavities from rock formations have involved driving a top tunnel from which drilling takes place. This necessitates the provision of a large number of drill holes and therewith an excessive charge of explosives, which places the roof of the cavity under unnecessary strain. Construction of the top tunnel also results in disturbance of the rock located above the cavity, with subsequent risk of impaired strength.
Upon discovery that microorganisms grow in the boundary layer between the product stored and the water present, it was demanded that any water present should be kept to a minimum, and it was further proposed in conjunction herewith that the cavity walls be coated with an imprevious composite lining comprising multi-layers of shotcrete, reinforced shotcrete, epoxy resin, glass fibre fabrics, and additionally epoxy resin. One such cavity-lining method is described by Beckers-Sigman, COLTURIET products.
It is not certain, however, that a lining of this nature would be able to provide durable protection should the lining be subjected constantly to water pressure on the rock side thereof. Consequently, in order to guarantee prolonged resistance of the lining, additional methods have been proposed for eliminating ambient water (SE-A-8300185-9).
It has been found that bench blasting has a highly deleterious affect on the residual cavity wall, and hence it is necessary, at high costs, to bolt the cavity wall and to line the same in order to achieve a durable result. Bench blasting also results in the formation of microfissures through which water in surrounding rock can enter the cavity.
Bench blasting also presents a serious risk to the working environment of those responsible for drilling the holes.
For reasons of a technical and environmental nature, there has now been raised a demand for a new method of excavating vertical rock cavities.